Use texture + color ramp to set density vertex group for particle system
Vertex groups are one method of controlling where particles generate, but there are also particle textures.
Create a texture from your color ramp (one way is to connect the color ramp node direct to material output, then bake). This should be an image that is full black where you don't want particles and full white where you do.
Then in the Textures panel of your particle system menu, press the + to add a texture. In the settings for the texture, under the Influence tab, check the box for Density.